Nick said:
Liquid CO2 won't hang around on the surface of Mars, it explodes.

(So all the channels must be formed by gas and debris clouds, in a CO2-based interpretation)

I magine something so much larger than the grand canyon with a howling storm of boiling CO2 gas, dust, sand, and rocks pouring down it at Mach 0.5...

My question(s):
That sounds like Hell on Mars, Nick! What a spectacle it would have been! What kind of time frame would you postulate for the channel formation? And where did all that huge volume of CO2 come from? What was it doing before the outburst? Would there be any kind of CO2 cycle to provide longer-term flow?